The question that puzzled them is ____ they can get rid of the air pollution in the area.

A．how is it that

B．that is how

C．what is that

D．how it is that

______能够实现对内外存进行统一管理，为用户提供一种宏观上似乎比实际内存容量大得多的存储器。

A．覆盖技术

B．交换技术

C．物理扩充

D．虚拟存储技术